The manufacture of dental prostheses such as crowns and bridges needs to be as precise as possible in order to ensure that, externally, the prosthesis fits within the area of the oral cavity assigned thereto, while also fitting properly onto the preparation. At the same time, it may sometimes be desired to have a number to of layers in order to provide the prosthesis with a natural-looking appearance, in which inner layers may be more opaque than outer layers, for example, or contain a mosaicing of differently colored patches, and/or wherein each layer may be made from a different material.
Manual manufacturing methods for dental prostheses typically require a number of “fitting and fixing” cycles in order to ensure that the crown is properly dimensioned before finally fixing the prosthesis to the preparation in the intraoral cavity.
CNC-based methods for manufacturing dental prostheses are known and represent a significant improvement in automating the manufacturing process to provide a high degree of dimensional accuracy. For example, in U.S. Pat. No. 4,663,720 and in U.S. Pat. No. 5,027,281, material is removed from a massive block of material by means of a CNC milling machine, and the machining paths are calculated from a 3D numerical model of prosthesis. In U.S. Pat. No. 4,937,928, a dental prosthesis is manufactured by successively applying a number of layers of prosthesis material on a model in the shape of the part of the teeth where the prosthesis is to be provided. After each layer is applied, the workpiece is worked by a CNC tool controlled by a CAD/CAM system. In U.S. Pat. No. 5,378,154, a similar method is used for forming layers of material onto a preparation, machining each layer along paths that follow three dimensional irregularly spaced curved lines.